Thanks Justin. Do you know if Postgres treats an UPDATE that sets the indexed columns set to the same previous values as a change? Or does it only count it as "changed" if the values are different. This is ambiguous to me.
> HOT solves this problem for a restricted but useful special case where a tuple is repeatedly updated in ways that do not change its indexed columns.
> With HOT, a new tuple placed on the same page and with all indexed columns the same as its parent row version does not get new index entries.
> [HOT] will create a new physical heap tuple when inserting, and not a new index
tuple, if and only if the update did not affect indexed columns.
On Thu, Nov 22, 2018 at 2:40 PM Justin Pryzby <pryzby@xxxxxxxxxxxxx> wrote:
On Thu, Nov 22, 2018 at 01:31:10PM -0800, Abi Noda wrote:
> In other words, is Postgres smart enough to not actually write to disk any
> columns that haven’t changed value or update indexes based on those columns?
You're asking about what's referred to as Heap only tuples:
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/access/heap/README.HOT;hb=HEAD
https://wiki.postgresql.org/wiki/Index-only_scans#Interaction_with_HOT
Note, if you're doing alot of updates, you should consider setting a lower the
table fillfactor, since HOT is only possible if the new tuple (row version) is
on the same page as the old tuple.
|With HOT, a new tuple placed on the same page and with all indexed columns the
|same as its parent row version does not get new index entries."
And check pg_stat_user_tables to verify that's working as intended.
Justin
